Brown dwarfs surprise

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We're up and running here at what President of the RAS, Prof Andy Fabian, described as super-NAM! Among the first series of presentations is the brown dwarfs and exoplanets session, which I'm sat in right now as I type this. Brown dwarfs (the first of which, Gliese 229B, was discovered in 1995) are often described as failed stars, at best a few dozen times as massive as Jupiter, but as Frances Allard of CNRS in Paris described in her opening talk, they can be as small as just a few times the mass of the planet Jupiter, which makes a bit of a mockery of the official definition of a body no smaller than 12 Jupiter masses. Exoplanets such as HAT-P-2b, which is nine times the mass of Jupiter, and CoRoT-3b with a mass 21 times that of Jupiter, seem to blur the boundaries between planet and failed star. So what makes a planet a planet and a brown dwarf a brown dwarf? 

Well, it seems they form differently, for a start; According to Allard, brown dwarfs, like stars, form from the core collapse of a gas cloud, whilst planets form from the bottom up in core accretion. Planets also have a much richer atmospheric composition, with much larger amounts of heavy elements and several convection layers. Brown dwarfs rotate rapidly and according to Allard's computer simulations, they have thick cloud cover swathing their atmospheres, full of the likes of ammonia and silicate dust. Breaks can occur in the cloud cover, resulting in variability in their brightness, not that they are very bright in the first place - their relatively low temperature (as 'cool' as 300 degrees Celsius) means they are dim and only visible at infrared wavelengths. Most astonishingly of all, they have magnetic fields likes stars, and in some cases they can be incredibly powerful: one brown dwarf, TVLM 513-46546, has been observed to flare at radio wavelengths with energies 1,000 times greater than the Sun. Who'd have thought these 'failed stars' could pack such a punch?

Image: An artist's impression of a low mass dwarf, courtesy NASA/ESA/G Bacon (STScI). 

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This page contains a single entry by Keith Cooper published on April 20, 2009 11:39 AM.

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